1. Field of the Invention
The present invention relates to switching power supply devices (switched-mode power supply device) of a non-linear control type.
2. Description of Related Art
FIG. 41 is a circuit block diagram showing one conventional example of a switching power supply device. Compared with switching power supply devices of a linear control type (e.g., a voltage mode control type and a current mode control type), switching power supply devices of a non-linear control type (e.g., a fixed on-period type, a fixed off-period type, and a hysteresis-window type) have the advantages of better load response combined with simple circuit configurations.
An example of conventional technology related to what is mentioned above is seen in Japanese Patent Application Publication No. 2014-017859 (hereinafter Patent Document 1).
On the other hand, according to a conventionally proposed function (for saving power under a light load), during the on-period of the synchronous rectification transistor N2, a reverse current toward the synchronous rectification transistor N2 is detected to turn off the synchronous rectification transistor N2. Inconveniently, however, once this power-saving function is activated, it takes time for the feedback voltage FB to fall below the reference voltage REF. This results in a low switching frequency under a light load, possibly causing noise audible to the human ear.
As a solution, according to another conventionally proposed function (for quiet operation), the switching frequency under a light load is kept above the human audible frequency range (roughly 20 Hz to 20 kHz) to suppress audible noise. Conventionally, such quiet operation is achieved in the following manner: when, after an on-timing of the output transistor N1, a predetermined threshold time elapses without the next on-timing coming, the synchronous rectification transistor N2 is forcibly turned on so as to lower the feedback voltage FB down to the reference voltage REF, thereby to invite an on-timing of the output transistor N1.
An example of conventional technology related to what is mentioned above is seen in Japanese Patent Application Publication No. 2014-075856.
Diminished Efficiency During Quiet Operation:
In the conventional quiet operation mentioned above, the synchronous rectification transistor N2 is forcibly turned on so that, as a result of the electric charge stored in the capacitor C1 being dumped to a ground node, the level shift timing of the main comparator is brought forward. This results in diminished efficiency.
Moreover, in the conventional quiet operation mentioned above, while a reverse current is just flowing from the coil L1 toward the synchronous rectification transistor N2, an on-timing of the output transistor N1 comes. Thus, turning the output transistor N1 only once does not allow sufficient energy to be stored in the coil L1, and consequently the feedback voltage FB may not rise above the reference voltage REF. In such a case, even under a light load, the output transistor N1 is held on throughout a plurality of cycles. This causes diminished efficiency.
Overshoot in Output Due to a Sharp Fall in Load:
Moreover, in a switching power supply device 100 of a non-linear control type, an overshoot occurs in the output voltage OUT in response to a sharp fall in load. To cope with it, various methods for suppressing an overshoot have conventionally been proposed.
According to a first conventional method, the output voltage OUT is monitored to check whether or not an overshoot is occurring, and according to the result of the check, an overshoot suppression function is activated. Inconveniently, however, with the first conventional method, the overshoot suppression function is not activated before an overshoot is actually detected. This results in a delay in the overshoot suppression action, diminishing its effect.
Improving the delay in the suppression action mentioned above requires a fast comparator. This invites increased cost and increased current consumption.
According to a second conventional method, when, as the load falls, the synchronous rectification transistor N2 is held on continuously for a predetermined period, an overshoot suppression function is activated. However, in the switching power supply device 100 of a non-linear control type, the switching frequency tends to vary according to operation status, and thus, with the second conventional method, an overshoot suppression function may be activated erroneously. For example, due to a restriction on the minimum on-time, producing a low output voltage OUT from a high input voltage IN causes the switching frequency to be low. This cannot be coped with by the second conventional method.
Diminished Efficiency Under a Light Load, and Erroneous Operation During Switching of Operation Modes:
Patent Document 1 discloses a switching power supply device provided with a sleep mode in which an on-time setting circuit and a reverse current detection circuit of the device are turned on and off as necessary.
To be sure, with the switching power supply device disclosed in Patent Document 1, going into sleep mode helps reduce the power consumption by the on-time setting circuit and the reverse current detection circuit, and this helps improve efficiency under a light load.
However, in the conventional switching power supply device, the highest priority is given to securing safety against, for example, a short circuit between output and ground: an overcurrent protection circuit is kept on all the time even under a light load, and a certain amount of driving current is consumed constantly. Thus, as for efficiency under a light load, there still remains room for further improvement.
Moreover, during switching between operation modes (during transition from normal mode to sleep mode, or during recovery from sleep mode to normal mode), the driving current for sleep target circuits varies greatly. Thus, with the conventional switching power supply device, the variation in current may cause unintended switching operation, possibly leading to increased output ripples.